Carphone Warehouse Return Policy on Pay As You Go Phones (Instore)?

I bought a cheap mobile from CPW yesterday with £10 credit on Virgin Mobile. I have now found the same phone cheaper online with free delivery and some quidco too. I went to store to find if I can return my phone back as it's less than 24 hours since I bought it. The guy at store says I can return the Mobile but Sim Credit will not be refunded! I have used the SIM infact I haven't even taken the sim out or unpacked it. I don't need the credit anyway, anyway I can get refund on this or can I exchange air time to another network of my choice?